The present invention relates to an optical path monitoring device on the basis of the identification of input ports in optical cross-connect systems.
The optical path is a logical path between two optical terminal devices sending and receiving optical signals in optical transport networks.
In optical transport networks, the optical path is established with the help of wavelength-division multiplexing optical communication technology.
FIG. 1 is a diagram illustrating an optical transport network in accordance with an embodiment of the present invention under the assumption that wavelength conversion is not occurred. As shown in FIG. 1, optical signals of wavelength 1, wavelength 2, and wavelength 3 are multiplexed and transmitted. The optical signals of the same wavelength are routed independently at each node and an optical path is established between two optical terminal devices. Thanks to wavelength-division multiplexing optical communication, one optical link can have more than 2 optical paths.
The optical transport network is reconfigurable, which reestablishes routing at nodes and copes with traffic congestion and optical link fault.
Since the optical transport network has transparency, it is able to work with new optical terminal devices and changes of transmission signal format.
The nodes of the optical transport networks employ optical cross-connect systems, which route optical signals with respect to wavelengths.
Optical paths at optical transport networks are determined by routing state of optical cross-connect systems. More particularly, the optical paths depend on switching state of optical switching device in the optical cross-connect system.
Problems occur in the whole network when the optical switching devices don""t work properly, which means it operates differently from the way the network management system commands.
Therefore, the optical paths of signals need to be monitored in order to increase the fidelity of optical transport network management.
The best way to monitor optical paths is to maintain current telecommunication standard as much as possible.
The conventional methods of monitoring optical paths are shown in FIG. 2, FIG. 3, FIG. 4a, and FIG. 4b. 
In FIG. 2, each optical terminal device superimposes unique pilot tone of low frequency on optical signals to be transmitted. The pilot tone frequencies are determined not to be affected by the signal modulation frequency. The notation xe2x80x98wavelength i (Pj)xe2x80x99 stands for the optical signal of wavelength i superimposed by pilot tone j.
The optical paths of optical signals are monitored by the pilot tone detector that is installed at each node and detects the pilot tone superimposed on the optical signals.
In order to transmit the pilot tone, the frequency of the pilot tone needs to be within the transmission bandwidth of optical amplifiers.
As the size of optical transport network gets bigger and the number of optical amplifiers in an optical path is more than one, the bandwidth that pilot tone can propagate gets smaller. The feasible bandwidth of pilot tone is seriously restricted and therefore the number of optical paths in an optical transport network is also limited.
Additionally, if a number of pilot tones are supplied to an optical amplifier at the same time, a new frequency component is generated because of the nonlinear effect of optical amplifiers. This can cause errors in pilot tone detection.
FIG. 3 is a diagram for illustrating a method monitoring optical paths using optical path overhead. Each optical signal includes optical path overhead and therefore finds out routing state of optical cross-connect systems.
However, a new transmission signal format different from the current standard needs to be determined.
FIG. 4a is a diagram for illustrating a method monitoring optical paths using amplified-spontaneous emission optical channel of optical amplifier.
The optical amplifier 410 compensates transmission loss of optical links. Amplified-spontaneous emission light of the optical amplifier 410 is passed through the Fabry-Perot filter 420 and feedback-amplified. The tunable Fabry-Perot filter is controlled so that its pass-band scans the amplified-spontaneous emission at the unique frequency assigned to each input port of optical cross-connect systems.
As shown in FIG. 4b, the output signal of optical cross-connect systems includes the amplified-spontaneous emission optical channels in addition to the input wavelength-division multiplexed signals. The power of each amplified-spontaneous emission optical channel is modulated at the same frequency as the scanning frequency of the Fabry-Perot filter, from which the amplified-spontaneous emission optical channel originates. The amplified-spontaneous emission optical channels are apart from the wavelength of wavelength-division multiplexed signals by free spectral range of wavelength-division demultiplexers.
Each amplified-spontaneous emission optical channel corresponds with an optical signal of each wavelength one by one and travels through the same optical path in optical cross-connect system.
The same number of fiber bragg gratings 440 as the amplified-spontaneous emission optical channels reflect the amplified-spontaneous emission optical channels and an optical circulator 430 directs the reflected the amplified-spontaneous emission optical channels to the frequency detector. The frequency detector finds out the modulation frequency of each amplified-spontaneous emission optical channel. Routing state of the optical cross-connect system is known from the modulation frequency of the amplified-spontaneous emission optical channel. Then, the routing state is compared with commands received from network management system, and optical paths of optical transport networks are monitored.
Because wavelength range of the amplified-spontaneous emission optical channel is different from the wavelength range of optical signals, transmission of the optical signals is not affected.
A band rejection optical filter 450 is used to completely remove the amplified-spontaneous emission optical channel and therefore the amplified-spontaneous emission optical channels with the same wavelength range can be reused in different optical cross-connect systems.
However, as the size of the optical transport network and the number of optical paths get increased, the wavelength range that amplified-spontaneous emission optical channels can use is significantly reduced with this method.
In addition, optical devices such as tunable Fabry-Perot filters, fiber bragg gratings, optical circulators, and band rejection optical filters are required and it causes additional cost.
An optical path monitoring apparatus in an optical cross-connect system is provided. The optical cross-connect system in accordance with the present invention includes input ports, wavelength-division demultiplexers, optical switching devices, a switching control device, optical power regulating devices, wavelength-division multiplexers, and output ports. The optical path monitoring apparatus in accordance with the present invention includes a plurality of pilot tone superimposers, a plurality of optical splitters, and a plurality of pilot tone detectors.
The plurality of pilot tone superimposers superimposes pilot tone on wavelength-division multiplexed optical signals and provides the superimposed signals to the wavelength-division demultiplexers. The superimposers are connected with the input ports.
The plurality of optical splitters splits optical signals that are provided by the optical switching devices. Each of the optical splitters is connected with each of the output ports of optical switching devices.
The plurality of pilot tone detectors receives one channel of optical signals, extracts pilot tone and thereby distinguishes which input port of the optical cross-connect system provides the optical channel, and detects optical path errors. The optical signals are split by the optical splitters.
Response time of the optical power regulating device is short enough to remove the pilot tone that is superimposed on the optical channel.
Desirably, the pilot tone superimposers include a wavelength-division multiplexing coupler, an active medium, a pump light source, and a current source.
The wavelength-division multiplexing coupler multiplexes input optical signals and pump light. The active medium absorbs pump light and amplifies optical signals. The pump light source provides energy to the active medium. The current source provides current to the pump light source. The current of the current source is modulated with pilot tone.
Desirably, the pilot tone superimposer includes an active medium and a current source.
The active medium receives current and amplifies optical signals. The current source provides current to the active medium. The current of the current source is modulated with pilot tone.
Desirably, the pilot tone superimposer is an accusto-optic modulator.
Desirably, the pilot tone superimposer is an integrated optical modulator.
Desirably, the pilot tone detector consists of a photo-diode and an electrical band-pass filter.
Desirably, the pilot tone detector consists of a photo-diode and a lock-in amplifier, and the lock-in amplifier uses the superimposed pilot tone as the reference signal.
Desirably, the optical power regulating device uses an accusto-optic modulator as an optical variable attenuator.
Desirably, the optical power regulating device uses an integrated optical modulator as an optical variable attenuator.